The comfort of users of an area is a function of several variables, including the following:
1. The activity level of the users of the area;
2. The clothing worn by the users of the area;
3. Air movement in the area;
4. The humidity level in the area;
5. The air temperature in the area; and
6. The mean radiant temperature, which indicates the combined effect of all surface temperatures in a room as those factors effect the heat gain and/or loss of the users of the room.
These factors are discussed in the text Thermal Comfort by P.O. Fanger (Robert E. Krieger Pub. Co. 1982).
The above factors are indicative of the predicted mean vote (PMV), a measure of mean thermal sensation among a large group of people under similar conditions. Such a measure is used because different people will perceive the comfort level differently even under identical conditions. The first two factors, the activity level and clothing of the users, are typically independent of a control system for the area to be controlled. The last four factors may be directly affected by the control system operation.
Typical control systems do not take all four of these environmental factors into account. Sensing and adjusting for environmental factors other than T.sub.mrt will tend to increase the comfort level for an environment such as a room. For example, relative humidity may be sensed to adjust the thermostat set point to maintain the mean thermal sensation among users of a room. When a temperature of a room is held constant, the room may feel warmer when relative humidity increases. Control systems may automatically reduce the thermostat set point when relative humidity increases to compensate for the increase in the mean thermal sensation experienced by users of a room when relative humidity increases.
However, failure to measure T.sub.mrt may result in inappropriate modifications of the environment. For example, the relationship between increases in relative humidity and increases in the mean thermal sensation may be misinterpreted in some climate conditions. The mean radiant temperature may increase, leading to a higher mean thermal sensation. However, if the increase in T.sub.mrt is accompanied by a decrease in the relative humidity, the operating set point will increase in a T.sub.mrt -independent system, leading to discomfort.
While it is desirable to sense the mean radiant temperature in the room and incorporate that value into the comfort control system, sensing the mean radiant temperature is difficult. The mean radiant temperature is a function of all the surface temperatures in a room. Thus, applying a temperature sensor to only one room surface will give only a partial indication of the mean radiant temperature for the room. Moreover, the T.sub.mrt is a function of the users' position within the room.
The Fanger text indicates that each surface temperature may be sensed, but that such a process requires a number of thermometers and a considerable amount of calculation work. T.sub.mrt may also be directly measured by use of a thermocouple or other temperature sensor inside a sphere. Such spheres are typically painted black. Copper spheres are used for their ability to spread temperature effects uniformly around the globe. However, copper spheres have a substantial time lag. Fanger suggested the use of a thin plastic bubble or balloon, but notes that spheres are generally unavailable on the market. Even when spheres are available, it may be difficult to locate the sphere in the room, especially without the sphere being quite noticeable. For some or all of these reasons, current control systems do not measure T.sub.mrt. T.sub.mrt may be approximated through the use of thermometers without spheres. However, while temperature sensors may be placed to indicate a temperature which is a function of both the air temperature and the mean radiant temperature, such efforts tend to distort the mean radiant temperature.